1. Field of the Invention
The present invention relates to a system and method for providing medium access control (MAC) protocol to direct transmissions in wireless devices such that specific absorption rate (SAR) limits are met. Specifically, the present invention uses “source-based” time averaging measurements of transmitter “on-times” and an integral of transmission power to direct transmitter duty-cycle such that SAR limits are met.
2. Description of the Related Art
Wireless communications networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive such data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base node to communicate simultaneously with several mobile nodes in it's coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed for use by the military. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks, such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, granted on Jul. 4, 2006, in U.S. Pat. No. 6,807,165 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, granted on Oct. 19, 2004, and in U.S. Pat. No. 6,873,839 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, granted on Mar. 29, 2005, the entire content of each being incorporated herein by reference.
A mobile node may include any number of devices, such as laptop computers or handheld mobile telephones. In the case of many of these devices, concerns regarding electric/magnetic fields have led to the creation of guidelines regarding optimum safe operation. The FCC is required by the National Environmental Policy Act of 1969 to evaluate the effect of emissions from FCC-regulated transmitters on the quality of the human environment. At the present time there is no federally-mandated radio frequency (RF) exposure standard. However, several non-government organizations, such as the American National Standards Institute (ANSI), the Institute of Electrical and Electronics Engineers, Inc. (IEEE), and the National Council on Radiation Protection and Measurements (NCRP) have issued recommendations for human exposure to RF electromagnetic fields. The potential hazards associated with RF electromagnetic fields are discussed in OET Bulletin 56 entitled, “Questions and Answers About the Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields”, the entire content of which being incorporated herein by reference.
As described in an FCC Bulletin entitled “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields”, Supplement C to OET Bulletin 65, the entire content of which being incorporated herein by reference, FCC rules require routine environmental evaluation of RF exposure for certain mobile and portable devices. Unless the device is categorically excluded from routine environmental evaluation, applications to the FCC for equipment authorization must include an affirmative statement indicating that the device is in compliance with FCC-adopted limits for RF exposures. In some cases it may be necessary for the applicant to provide certain information to document the test procedures used to evaluate compliance.
Mobile devices that operate at 1.5 GHz or below with an effective radiated power (ERP) of 1.5 watts or more, and those devices that operate at frequencies above 1.5 GHZ with an ERP of 3 watts or more are required to perform routine environmental revaluation for RF exposure prior to equipment authorization or use. Mobile devices may be evaluated with respect to field strength, power density or specific absorption rate (SAR) limits. Devices may be evaluated with respect to SAR limits using either measurement or computational methods.
The Federal Communications Commission (FCC) sets SAR limits for handheld and portable devices which are used close to human bodies and do not pass electric/magnetic field limits due to the close proximity of device antennas to the human body. The FCC rules for evaluating portable devices for RF exposure compliance are described in 47 CFR §2.1093, the entire content of which being incorporated herein by reference. As noted in the FCC Bulletin entitled “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields”, referenced above, for purposes of RF evaluation, a portable device is defined as a transmitting device designed to be used with any part of it's radiating structure in direct contact with the body of the user or within 20 centimeters of the body of the user under normal operating conditions. This category of devices would include hand-held cellular and PCS telephones that incorporate the radiating antennae into the handpiece and wireless transmitters that are carried next to the body. Portable devices are evaluated with respect to SAR limits for RF exposure. For most portable transmitters used, the applicable SAR limit is 1.6 watts/kg as averaged over any one gram of tissue.
Protection guidelines typically include operating warnings and warning labels for specific operation ranges, as described in an FCC Bulletin entitled “Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields”, Supplement C to OET Bulletin 65, referenced above. When operating and warning instructions are ineffective, the use of warning labels on the transmitting element may also be necessary to caution nearby persons to limit their exposure duration and/or conditions to ensure compliance. If warning labels are not desirable, specific absorption rate evaluations, even when not required, may be used to demonstrate compliance to obviate the need for any warning label that might otherwise be necessary. Additional details of specific absorption rates may be found in 47 CFR 2.1091, and 47 CFR 2.1093, the entire contents of each being incorporated herein by reference.
However, depending on antenna gain and transmission power levels at a device, SAR requirements can typically be difficult with which to comply. Evaluation of mobile devices may be performed with respect to compliance with SAR limits, but it is often simpler and more cost-effective to evaluate compliance with respect to field strength or power density. Accordingly, a need exists for a medium access control (MAC) protocol which directs and controls transmitter operation at a device, in an ad-hoc system with large cell radius, such that SAR requirements are satisfied and demonstrate compliance to obviate the need for any warning label that might otherwise be necessary.